My dad used to dust our yard with powdered sulphur to encourage the chiggers to find another home. The ones that got us anyway we put clear nail polish on. If you've ever tried ChiggeRid, that's all it is, just clear nail polish at twice the price (don't know if they still sell it).
I was told that they got the back of your knees and your elastic lines because they crawled up till they hit something that blocked their way, then they dug in...